1 {photo of dozens of beetles}
1 Caption: Animal diversity

The top level of taxonomic organisation within the animal kingdom is the grouping known as the phylum (plural: phyla). Animals are classified into a phylum based on the highest level anatomical features shared across groups of creatures.

Phyla are bigger and more inclusive than you might expect. Humans, for example, are in the same phylum as dogs and cats, and mice and rats, and kangaroos and koalas - and also birds and snakes and frogs and fish. All mammals, plus birds, reptiles, amphibians, and fish, including the cartilaginous fish such as sharks and rays, are grouped together into the group known as Vertebrata, the vertebrates, or animals with bones. But even this large grouping is not as big as the phylum which contains it. Also in the same phylum are animals such as the lancelets, a group of small eel-like creatures without a skeleton, and sea squirts, small squishy creatures which you might find in rock pools by the sea and which emit squirts of water when disturbed.

All of these animals make up the phylum named Chordata. The name refers to the notochord, a sort of proto-backbone, which runs down the body of each creature in these families, at least at some stage during their development. In vertebrates, the notochord eventually becomes encased in bone or cartilage, forming the basis of the spine. But in the lancelets and sea squirts, the notochord remains a flexible rod of stiffening tissue, bare of any bone. The first chordates were floppier creatures which evolved this stiffening rod running through their bodies. This aided movement, conferring the advantage which saw it selected and propagated through successful descendants, resulting in the wide range of animal types today which share this same basic body plan. This shared ancestry makes all the animals of the phylum Chordata related in a very fundamental way.

This one phylum, Chordata, covers most of the animals you probably think of when you think of the word "animal". It includes all animals with bones, and a few other things besides. But there are several other phyla.

Eastern water skink, a representative of Chordata.

The next most obvious one is represented by all the various insects we find around us. But again, despite the wide diversity of insects, their phylum is much more inclusive. Grouped together with insects in the same phylum are spiders, scorpions, centipedes and millipedes, as well as crabs, lobsters, prawns, barnacles, and all the other crustaceans. This large group of creatures makes up the phylum Arthropoda, the arthropods, which is distinguished by a body structure with a jointed exoskeleton. There is a huge difference between the fundamental body structure of chordates, with an internal supportive structure, and arthropods, with their soft internal bodies, hard carapaces, and segmented structure to allow movement.

Sally lightfoot crab, a representative of Arthropoda.

Now might come to mind a few animals which don't fit into either of these two large phyla. Snails and slugs don't have bones, or the all-covering exoskeleton of insects or crustaceans. Indeed, they are representatives of a third great phylum of animal life, which also includes sea snails, oysters, clams, mussels, nautiluses, squid, and octopus. These are the molluscs, which form the phylum Mollusca. They are distinguished by a multi-layered body covering, encasing muscular body tissue and multiple differentiated organs, but without bones or exoskeleton. The body covering is called the mantle and forms a sort of drape over the rest of the body, often extended into various flaps of "skin" that perform functions such as locomotion, feeding, or breathing either air or water. Some molluscs excrete shells, made primarily of calcium carbonate, but these are different to the form-fitted exoskeletons of the arthropods, which are made of protein. A few molluscs, such as the cuttlefish, have calcium carbonate "shells" within their bodies - this is the so-called cuttlebone which can sometimes be found washed up on beaches, but which is not the same thing as the bones of vertebrates.

Green-lipped mussels, a representative of Mollusca.

These three phyla alone account for most of the animals that people are familiar with, but there are a few odd ones out remaining. Starfish don't fit into any of the above three phyla: no bones or notochord, no exoskeleton, and no mantle. They also have a five-pointed star shaped symmetry, while animals in the three phyla described so far have a bilateral left-right symmetry. And while not having any supportive bones, starfish have a collection of hard bony spines embedded in their skins. All of these features are shared with sea urchins, sea cucumbers, sand dollars, and crinoids (also known as sea lilies). Together, this group of creatures comprise the phylum Echinodermata, the echinoderms (meaning "spiny skin").

Starfish in a pool, representatives of Echinodermata.

A few well known animal types still remain unaccounted for. Jellyfish share their circularly symmetrical bodies and tentacles loaded with stinging cells with sea anemones, coral polyps, and small creatures known as hydras (based on their shape resembling the mythical Hydra). These jelly filled creatures make up the phylum Cnidaria.

Pacific sea nettles, representatives of Cnidaria.

The last animal phylum for which there are commonly familiar representatives encompasses the earthworms, leeches, and similarly structured, segmented round worms. These are the Annelida, or annelid worms. As these sentences imply, there are other sorts of less commonly considered worms which are not included in Annelida, but we'll get to those in a moment.

Another phylum is Brachiopoda, the brachiopods. These animals could easily be mistaken for bivalve molluscs such as clams, as they have a pair of shells. However, bivalve shells emerge from the left and right side of the animal's body, while brachiopod shells emerge from the upper and lower surfaces, and do not enclose the entire animal. A long tubular stalk emerges behind the shells and is used to anchor the brachiopod to the sea floor like an anemone. There are other anatomical distinctions between brachiopods and molluscs involving the positions and embryonic development of various organs, forming enough of a distinction in body structure that the brachiopods get their own phylum.

Sponges are animals with virtually no body structure at all. In fact, you can mash a sponge through a sieve, and the cells that pass through will re-merge into an intact sponge on the other side, with no harm done. Sponges have no organs, but they do have a body structure formed of two layers of cells enclosing a jelly-like layer of proteins called the mesohyl. They filter water through pores in their bodies to capture drifting particles of food, having neither mouths not digestive tracts. Most sponge species have an internal supportive structure made of collagen, which gives them a shape. It is this protein structure which remains after a sponge dies and the cells decay, which gives us the absorbent daubing tool which we also call a sponge. The sponges as a group of animals form the phylum Porifera.

Various sea sponges, representatives of Porifera. Creative Commons Attribution-NonCommercial-Share Alike image by John Turnbull.

Now we come to the many other types of worms which inhabit our planet, beyond those of the Annelida. A worm-like body is a simple and effective shape for many modes of life, so many different types of creatures have evolved such a shape, which looks very similar to our quadruped-biased eyes. But when examined closely, the bodies of different types of worms exhibit a wide variety of structures that reflect their different origins and divergent evolutionary paths. These are easily enough to define several different phyla, each one dedicated to worms of a particular specialised body structure.

Probably the next best known phylum of worms after Annelida is Platyhelminthes, the flatworms. This is a relatively large phylum which includes marine flatworms (which can be fairly large and attractive animals, swimming with a rippling body motion), freshwater flatworms, other species which live in moist environments such as leaf litter, and a host of parasitic worms such as tapeworms. Then there is Nematoda, the phylum of nematode worms, which I have talked about briefly before. Nematodes have simple round body structures and are possibly the most successful worm phylum, with thousands of free-living and parasitic species.

Marine flatworm (I don't know what species), a representative of Platyhelminthes.

And there are several other phyla of worms which are more or less unfamiliar to anyone but biologists: Acanthocephala, the thorny headed worms; Chaetognatha, the arrow worms; Entoprocta, the goblet worms; Gnathostomulida, the jaw worms; Hemichordata, the acorn worms; Nematomorpha, the horsehair worms; Nemertea, the ribbon worms; Onychophora, the velvet worms; Phoronida, the horseshoe worms; and Sipuncula, the peanut worms.

This is still not all of the animal phyla. There are groups of other bizarre creatures such as Tardigrada, the so-called water bears; Rotifera, the rotifers; Ctenophora, the comb jellies; Kinorhyncha, the mud dragons, and a few others as well.

Comb jelly, a representative of Ctenophora. Creative Commons Attribution-NonCommercial-Share Alike image by Keith Garner.

All of these phyla have been around for a very long time. They appeared during the Cambrian period, around 500 million years ago. This was, geologically speaking, shortly after the appearance of multicellular creatures. Life began on Earth very early - there are multiple lines of evidence pointing to the existence of unicellular microbes from pretty much as soon as the forming Earth had cooled enough to be able to support them, around 3.5 billion years ago. And for 3 billion years life remained a collection of one-celled organisms, harnessing energy from sunlight and chemicals from the environment to build their bodies.

But in the Cambrian all that changed. Some cells stuck together and began sharing the fruits of their activities. This made them more efficient and so clumps of cells began appearing, forming the first multicellular bodies. The world was wide open to exploitation by these new forms of life, and they branched out into numerous diverse forms, trying all sorts of body shapes and functions. This gave rise to the different phyla. As the ecological space became more crowded, the changes in body shape and function became less drastic, drawing on the success of ancestral forms rather than staking out new territory which other animals had already claimed. And so the diversity within each phylum was a bit less striking than the broad differences between the phyla. Given how populated the Earth is now, it would be very difficult, if not impossible, for a new phylum of life to emerge without immediately being eaten and cut short before it can diversify and become successful.

Gogia spiralis, a fossil from the Cambrian. Creative Commons Attribution-NonCommercial-Share Alike image by Steve Lew.

This relatively quick profusion of different life forms (it actually took millions of years) is known as the Cambrian explosion. We know about it from rare fields of fossils in which the shapes of soft body parts of these early animals are preserved. We know a lot more about the palaeontology of chordates and arthropods than any other phylum, because these creatures have hard parts which fossilise well: bones for chordates and exoskeletons for arthropods. Mollusc shells can be fossilised too, but more often they are squashed together and compressed over millions of years into calcium carbonate rocks like chalk, limestone, and marble. The soft bodies of other phyla are more likely to decay and only become fossilised in unusual circumstances. But fortunately a few deposits of fossil Cambrian fauna are known, and tell us the story of the ancestors of the animals all around us today.